Various types of semiconductor devices are manufactured in much the same way. A starting substrate, usually a thin wafer of silicon or gallium arsenide, is masked, etched, and doped through several process steps, the steps depending on the type of semiconductor devices being manufactured. This process yields a number of die on each wafer produced. The die are separated with a wafer saw, and then packaged into individual components.
During the packaging process, several semiconductor die are attached to a lead frame, often with materials such as conductive epoxy, various metals and alloys, or other adhesives. Bond wires electrically connect (i.e. couple, directly or through intermediate paths) a number of bond pads on each die to conductive lead "fingers" on the lead frame. Leads are interposed between the lead fingers and the host into which the device is installed. The die, the wires, and a portion of the leads are encapsulated in plastic. The leads on the lead frame connect the die with the device into which the component is installed, thereby forming an electrical pathway and a means of input/output (I/O) between the die and the host.
The particular lead finger with which a bond pad is connected determines the pinout for that bond pad. For example, in a dynamic random access memory (DRAM) if a bond pad on the die which corresponds to Address 0 (A0) is bonded to the lead finger corresponding to Output Pin 5, then pin 5 on the package is used as A0. This hardwires the bond pad on the die to the output of the lead frame, and remains that way for the life of the package. Once the die is encapsulated, the output pins for the signals required to operate the die cannot be changed.
In addition to providing external access to standard input and output signals, the wire bond step may also be used to select various optional operating features of an integrated circuit (IC) product (such as a DRAM). In the case of a DRAM IC, device data width may be selected at the wire bond step thereby determining whether the die is written to and read from, for example, 1 or 4 bits at a time. A 4 megabit (Mbit) device, therefore, can be configured as a 4Mbit.times.1 or a 1Mbit.times.4, depending on how the bond pads are wire bonded to the lead frame.
Fast page mode is another option which might be selected at wire bond. Fast page mode allows for two or more successive reads or writes from the same row without requiring another row address strobe (RAS) signal. For example, if RAS is kept low after a read, another cell or plurality of cells in the row can be read by issuing a different address on the address lines and then toggling the column address strobe (CAS), thus executing faster memory access cycles.
Similarly, static column mode might be selected at the wire bond stage. Static column mode is similar to fast page mode, but both RAS and CAS remain low, and a new address is presented to the address lines in order to read from or write to a different address in the row. Various other device modes and options are selected during the wire bond step. Sometimes a bond pad is not connected with an output lead, and the option is not selected.
One problem with selecting options by wire bonding an option into a package is that once the die is encapsulated the package options cannot be changed. Various customers require a different combination of options or a number of different package options. The manufacturer must either assemble the package as the order is placed, or the manufacturer or buyer must keep a stock of each of the various device types. Assembling the devices as they are ordered implies long lead times, while keeping a large stock increases operating costs of the manufacturer or the buyer.
Devices have been designed which allow for the selection of bond options after packaging of the die using an electrical in-package late programming technique. To fabricate this type of part, the die is attached to the lead frame and the die is encapsulated in plastic. Circuitry on the die allows the bond option to be electrically selected. Using this technique, however, only options that are compatible with the package pin count in which the part was assembled are selectable.
A package which allows the semiconductor manufacturer or buyer to package a die and configure bond options and pin counts as they are required would help solve the problems listed above. Note that even though many of the devices used as examples herein specifically mention die encapsulated in plastic, similar problems and solutions are workable for die housed in ceramic or other package materials.